It has been demonstrated that CD8 positive cytotoxic T lymphocytes (CTLs) recognize epitope peptides derived from the tumor-associated antigens (TAAs) found on the major histocompatibility complex (MHC) class I molecule, and then kill the tumor cells. Since the discovery of the melanoma antigen (MAGE) family as the first example of TAAs, many other TAAs have been discovered primarily through immunological approaches (NPLs 1, 2). Some of these TAAs are currently undergoing clinical development as immunotherapeutic targets.
Favorable TAAs are indispensable for proliferation and survival of cancer cells. The use of such TAAs as targets for immunotherapy may minimize the well-described risk of immune escape of cancer cells attributable to deletion, mutation, or down-regulation of TAAs as a consequence of therapeutically driven immune selection. Accordingly, the identification of new TAAs capable of inducing potent and specific anti-tumor immune responses, warrants further development and clinical investigation of peptide vaccination strategies for various types of cancer (NPLs 3-10). To date, there have been several clinical reports of trials using these TAA derived peptides (NPLs 11-13). Although some success has been observed, there remains a need for new TAAs as immunotherapeutic targets.
MELK, maternal embryonic leucine zipper kinase, has been previously identified as a new member of the snf1/AMPK serine-threonine kinase family that is involved in mammalian embryonic development (NPL 14). This gene has been shown to play an important role in stem cell renewal (NPL 15), cell-cycle progression (NPL 16, 17) and pre-mRNA splicing (NPL 18). To that end, through gene expression profiling with a genome-wide cDNA microarray containing 23,040 genes, the present inventors have been identified MELK which is up-regulated in breast cancer (NPL 19).
MELK is up-regulated in several cancer cells, for example, lung, bladder, lymphoma and cervical cancer cells. Northern blot analysis on multiple human tissues and cancer cell lines demonstrated that MELK was overexpressed at a significantly high level in a great majority of breast cancers and cell lines, but was not expressed in normal vital organs such as heart, liver, lung and kidney. Furthermore, suppression of MELK expression by siRNA has significantly been shown to result in growth of human breast cancer cells.
Multiple investigations has been reported on modifying the amino acid residue of the peptides that are crucial for the interaction with the MHC or the T cell receptor to enhance the immunogenicity of the peptides (NPL 20, 21).